1. Field of the Invention
The present invention relates to a steel sheet for hot press forming having low-temperature heat treatment property, a method of manufacturing the same, and a method of manufacturing parts using the same, and more particularly, to a steel sheet for forming, in which heat treatment is performed within a range of lower temperature than a conventional steel sheet in the event of hot press forming or post-heat treatment after cold forming, thereby making it possible to solve various problems occurring when the heat treatment is performed at high temperature and to secure sufficient strength, a method for manufacturing the same, and a method of manufacturing impact and structural parts for a motor vehicle.
2. Description of the Related Art
Recently, as motor vehicle safety regulations for passenger protection as well as fuel efficiency restrictions for global environmental protection have become strict, an increasing interest has been taken in the motor vehicle, particularly increasing its rigidity and reducing its weight. For example, in the case in which a variety of parts, such as pillar reinforcements or cross members for a safety cage zone surrounding a passenger compartment, side members for a crash zone, a front or rear bumper, etc., are focused on reducing weight, a high strength steel sheet is inevitably employed in order to secure both the rigidity and the safety against collision.
However, increase of the strength of the steel sheet for the motor vehicle high inevitably leads to problems in that formability is remarkably degraded due to increase in yield strength and decrease in elongation, and in that the dimensions of the parts are changed after forming due to excessive springback, namely the shape fixability is degraded. In order to solve these problems, there have been developed and commercialized the advanced high strength steels (AHSS) such as dual phase (DP) steels having low yield ratio property by introducing a martensite phases into a ferrite matrix and transformation induced plasticity (TRIP) steels having an extremely excellent strength-elongation balance by containing bainite and residual austenite phases in a ferrite matrix as well. And these steels having tensile strength between about 500 MPa and about 1000 MPa have been commercialized. However, they have limitations to meet the requirements of the strength moreover than 1000 MPa of the motor vehicle, i.e. reducing the weight and improving the safety against collision.
Meanwhile, from the viewpoint of the manufacturing of the motor vehicle parts, the higher the strength of material becomes, the higher the forming force is required. Thus, it is necessary to increase capacity of a press. Further, the productivity is lowered due to increased wear and decreased life span of dies caused by high contact pressure. A recently introduced method is called roll forming capable of manufacturing the parts with the forming force less than that of the press forming. However, this roll forming has a problem in that it can be applied only to the parts having a relatively simple shape.
As a method of manufacturing motor vehicle parts having ultra high strength of 1000 MPa or more in order to solve this problem with the formation of the high strength steels, a forming method called hot press forming or hot forming has been commercialized. This forming method is carried out by blanking a steel sheet having tensile strength ranging from 500 MPa to 700 MPa, heating the blanked steel sheet up to an austenite region above Ac3, extracting the heated steel sheet, forming the extracted steel sheet using a press equipped with the die which has cooling system, and die-quenching the formed steel sheet. Thereby, either martensite phases or phases in which martensite and bainite are mixed are finally formed. Thus, such a forming method is a method that can typically obtain the ultra high strength of 1000 MPa or more as well as very high dimensional precision of the parts.
Basic concept of the hot press forming and compositions of the steel used are proposed first in GB1490535. Afterwards, a hot- and cold-rolled steel sheet coated with aluminum or aluminum alloy restricting upper and lower limits of each element, but composition system similar to previous patent, is proposed in U.S. Pat. No. 6,296,805 to inhibit an oxide from forming on the surface of the steel sheet during heating in a hot press forming process. Further, a method of manufacturing hot press formed parts using a galvanized steel sheet which is manufactured by coating a hot rolled steel sheet with zinc or zinc alloy in order to improve corrosion resistance and to inhibit formation of an oxide layer in a heating process, is proposed in EP1143029. Further, Korean Patent Application Publication No. 2002-0042152 discloses a method of manufacturing a galvanized steel sheet for hot press forming.
However, as described above, the conventional steel sheets for the hot press forming are heat-treated steel sheets having a composition system in which titanium and chromium are added in common on the basis of a composition system of 22MnB5, i.e. 0.22% of C-1.2% of Mn-maximum 50 ppm of B, specified in EN standards. In order to obtain tensile strength of about 1500 MPa after heat treatment, it is necessary to typically heat these steel sheets at a temperature of 900° C. or more. However, the thinner the hot press formed part becomes, the faster the temperature of the blank extracted from a heating furnace is lowered. Thus, a possibility that the strength of a final hot press formed part is to be reduced is increased. In other words, if any material becomes thin, radiant heat capacity thereof is increased. As such, before the hot press forming is carried out after the blank is extracted from the heating furnace, cooling has already occurred excessively, and thus a possibility that ferrite is to be formed on a superficial layer is increased. For this reason, the strength of the final part is reduced. In contrast, in order to maintain the temperature of the entire material within the austenite region when the hot press forming is carried out, heating temperature must be additionally increased. However, if the heating temperature is increased, various problems additionally take place as follows. In detail, in the case of hot rolled steel sheets or cold rolled steel sheets, the thickness of a superficial oxide scale is increased during heating, the scales stripped off by the hot press forming are picked up on the surface of die, and thus the surface quality of the final part can be deteriorated.
In addition, in the case of galvanized steel sheets, when the steel sheet is heated, some of zinc is evaporated. In order to prevent this evaporation, JP2003-073774 discloses a method of forming a zinc oxide barrier layer during heating for the hot press forming. However, as described above, when the heating temperature is increased, the zinc oxide layer is non-uniformly formed, and thus the surface quality of the final part is also deteriorated. Further, in the case of aluminum coated steel sheets, when the heating temperature is increased, the thickness of aluminum oxide is increased. Further, while the hot press forming is carried out, there is a high possibility that the thickened aluminum oxides are stripped off and picked up on the die surface. Consequently, in the case of any steel sheet used at the hot press forming, when the heating temperature is increased, the surface quality of the final part is deteriorated. In addition, the heating cost is increased.
Further, in the case of a method of performing post-heat treatment in order to improve strength of the steel sheet going through the cold press forming rather than the hot press forming, it is preferable to decrease the heating temperature from the viewpoint of the cost of production.